The invention relates to a receiving apparatus suitable for Eureka DAB (Digital Audio Broadcasting) System, in particular, to an improvement on an influence of a multi-path to a timing detection signal.
As an example of digital audio broadcast systems, Eureka DAB System developed by European Eureka 147 Project is known. In Eureka DAB System, MPEG (Moving Picture Experts Group) layer 2 is used as an audio encoding method. In addition, OFDM (Orthogonal Frequency Division Multiplexing) is used as a modulating method. With such methods, Eureka DAB System broadcasts six channels of high quality stereo programs and one channel of data in a transmission band of 1.50 MHZ.
FIG. 1 shows the structure of the transmitter side of Eureka DAB System. In FIG. 1, audio data is supplied to an input terminal 101. The audio data is supplied from the input terminal 101 to an audio encoder 102. The audio encoder 102 compresses the audio data.
In Eureka DAB System, as a compressing process of audio data, MPEG layer 2 is used. The MPEG layer 2 audio compressing method is a sub-band encoding method in which an input signal is divided into a plurality of frequency bands and each divided signal is independently encoded. In other words, input audio data is divided into 32 sub-bands with a bandwidth of 750 Hz by an analyzing filter bank. In addition, an FFT (Fast Fourier Transform) process is performed for the input audio data so as to analyze individual components of these bands. Corresponding to the result of the FFT process, masking is calculated and bits are assigned to the individual bands.
An output signal of the audio encoder 102 is supplied to a channel encoder 103. The channel encoder 103 performs an encoding process for the compressed audio data with an error correction code such as a convolutional code.
An output signal of the channel encoder 103 is supplied to a time interleaving circuit 104. The time interleaving circuit 104 interleaves the signal received from the channel encoder 103 in the time direction. An output signal of the time interleaving circuit 104 is supplied to a multiplexer 105.
General data is supplied to an input terminal 106. The general data is information such as weather information and traffic information. General data received from the input terminal 106 is supplied to a data encoder 107. The data encoder 107 arranges the data received from the input terminal 106 in a predetermined format. An output signal of the data encoder 107 is supplied to a channel encoder 108. The channel encoder 108 performs an encoding process for the signal received from the data encoder 107 with an error correction code such as a convolutional code. An output signal of the channel encoder 108 is supplied to a time interleaving circuit 109.
The time interleaving circuit 109 interleaves the signal received from the channel encoder 108 in the time direction. An output signal of the time interleaving circuit 109 is supplied to a multiplexer 105. Thus, the multiplexer 105 multiplexes the audio data received from the terminal 101 with the general data received from the terminal 106.
An output signal of the multiplexer 105 is supplied to a frequency interleaving circuit 111. An FIC generating circuit 115 supplies FIC (Fast Information Channel) data to the frequency interleaving circuit 111. The frequency interleaving circuit 111 interleaves the output signal of the multiplexer 105 and an output signal of the FIC generating circuit 115 in the frequency direction. An output signal of the frequency interleaving circuit 111 is supplied to an OFDM circuit 112. A sync generating circuit 113 supplies a TFPR signal and a null symbol to the OFDM circuit 112. Thus, the TFPR signal and null symbol are added to the output signal of the frequency interleaving circuit 111.
The OFDM (orthogonal Frequency Division Multiplexing) method is a multi-carrier modulating method using a plurality of carriers that are perpendicular to each other. The OFDM circuit 112 correlates digital data with data in frequency region and performs an IFFT (Inverse Fast Fourier Transform) process so as to convert data in frequency region into digital data in time region.
An output signal of the OFDM circuit 112 is obtained from an output terminal 114. The output signal is modulated by xcfx80/4 QPSK modulating method and converted into a signal with a predetermined transmission frequency.
FIG. 2 shows the structure of a transmission frame transmitted in Eureka DAB System. As shown in FIG. 2, at the beginning of the transmission frame, a null symbol is disposed (thus, the relevant RF signal is not transmitted). The null symbol is used to coarsely synchronize with the received signal. The null symbol is followed by a TFPR signal. The TFPR signal is a reference symbol for controlling the frequency and synchronization of the received signal. The TFPR signal is followed by FIC (Fast Information Channel) data. The FIC data is control data that includes service information, display data, program service label, time and date, broadcast station ID, presence/absence of simultaneous service, and broadcast information of other channels. The FIC data is followed by MSC (Main Service Channel) data. The MSC data is general data such as music data, weather information, traffic information, and program list.
Eureka DAB System broadcasts signals through a satellite and VHF ground broadcasting stations. In Europe, the following three bands of frequencies have been assigned to Eureka DAB System.
Band 1 (47 MHz to 68 MHz)
Band 3 (174 MHz to 240 MHz)
L Band (1.452 GHz to 1.492 GHz)
In addition, depending on a frequency for use, as a hybrid method, both ground broadcasting stations and a satellite can be used.
When a mobile substance such as an automobile receives a signal of a conventional FM broadcast or a conventional AM broadcast, the signal is subject to fading due to an interference of a wave reflected by a building or the like to a direct wave. In addition, since the signal is analog, the sound quality is not high and the signal is subject to noise.
On the other hand, in Eureka DAB System, since an audio signal which is transmitted is a digital signal, the audio signal quality is high and the audio signal is not subject to noise. In Eureka DAB System, the OFDM method is used. In the OFDM method, since data is allotted to many carriers, the duration of one symbol is long. In the OFDM method, since a guard interval is placed on the time axis, even if a multi-path of which the delay time of a reflected wave is smaller than the interval is present, the transmission characteristic does not deteriorate. Since the waveform of a digital signal of the OFDM method has a resistance to random noise, the signal does not adversely affects other communications. In addition, the signal is not adversely affected by other communications. When carriers are allocated at particular intervals on the frequency axis, a frequency interleave effect is substantially obtained. In a combination with an adequate error correction code, the signal has a resistance to frequency selective fading.
In Eureka DAB System, in addition to high quality of an audio signal, multi-lingual broadcasts and general information such as weather information and traffic information are available. In Eureka DAB System, general information can be used for paging and still pictures can be transmitted as general information.
By the way, as shown in FIG. 2, in each transmission frame used in Eureka DAB System, a null symbol is disposed at the beginning. The null symbol is followed by a synchronous TFPR signal. With the TFPR signal, the frequency and timing of the having received signal or the receiving signal are controlled.
FIG. 3 shows the structure of a channel decoder of a conventional receiver for Eureka DAB System. In FIG. 3, an intermediate frequency signal is supplied to an input terminal 131. The signal received from the input terminal 131 is supplied to an A/D converter 132. The A/D converter 132 digitizes the signal received from the input terminal 121 into a digital signal.
An output signal of the A/D converter 132 is supplied to an I/Q demodulating circuit 134 and a sync generating circuit 135 through a band-pass filter 133. The I/Q demodulating circuit 134 orthogonally detects the received signal and demodulates it into an I signal and a Q signal. Output signals of the I/Q demodulating circuit 134 are supplied to an AFC (Automatic Frequency Controlling) circuit 136.
The sync generating circuit 135 detects a null symbol and receives a TFPR signal. An output signal of the sync generating circuit 135 is supplied to an AFC circuit 136.
An output signal of the AFC circuit 136 is supplied to an FFT circuit 139. An output signal of the FFT circuit 139 is supplied to a Viterbi decoder 140. In addition, the output signal of the FFT circuit 139 is fed back to the AFC circuit 136. The FFT circuit 139 and the viterbi decoder 140 are controlled by a timing controlling circuit 141.
The AFC circuit 136 detects a null symbol, receives a TFPR signal corresponding to the null symbol, performs the FFT process for the TFPR signal, obtains a frequency error of the received signal corresponding to the result of the FFT process, and controls the frequency and timing of the received signal corresponding to the frequency error. In other words, the sync generating circuit 135 obtains the TFPR signal corresponding to the null symbol. The TFPR signal is supplied to the FFT circuit 139. The FFT circuit 139 performs the FFT process for the TFPR signal. The resultant signal is fed back from the FFT circuit 139 to the AFC circuit 136. The AFC circuit 136 obtains a frequency error and a timing error of the received signal corresponding to the signal of which the FFT process has been performed for the TFPR signal. Corresponding to the frequency error and the timing error, the frequency and the timing of a carrier of the having received signal or the receiving signal are controlled.
The FFT circuit 139 performs the FFT process for the OFDM signal so as to demodulate the OFDM signal to the original data which is data before transmitting. In Eureka DAB System, when data is transmitted, digital data is correlated with data in frequency region. Data in frequency region is converted into data in time region by the IFFT process. With a plurality of carriers that are orthogonal to each other, data is transmitted. The FFT circuit 139 demodulates the OFMD signal to the original data. The FFT circuit 139 maps the received data so as to perform the FFT process. Thus, the received signal, that is data in time region is converted into data in frequency region and demodulated to base band data.
An output signal of the FFT 139 is supplied to the Viterbi decoder 140. In addition, the output signal of the FFT 139 is fed back to the AFC circuit 136. The Viterbi decoder 140 performs a maximum likelihood decoding process for a convolutional code so as to perform an error correcting process. The FFT circuit 139 and the Viterbi decoder 140 are controlled by the timing controlling circuit 141. An output signal of the Viterbi decoder 140 is obtained from an output terminal 142.
In Eureka DAB System, with the TFPR signal, the frequency and timing of a carrier of the having received signal or the had received signal are controlled. However, the TFPR signal which is the reference for controlling the frequency and timing of the received signal may be affected by a multi-path.
In other words, as shown in FIG. 4, when a mobile substance 151 such as automobile and so on receives a digital audio signal of Eureka DAB System, there may be paths P2 and P3 of waves reflected by buildings and so forth as well as a path P1 of a direct wave. When there is a multi-path as shown in FIG. 5, the TFPR signal is received as signals S12 and S13 through the paths P2 and P3 of the reflected waves as well as a signal S11 through the path P1 of the direct wave. When the TFPR signal is received through the multi-path, it becomes difficult to correctly control the frequency and timing of the received signal.
Therefore, an object of the present invention is to provide a receiving apparatus that allows an influence of a multi-path to a synchronous reference signal to be removed.
A receiving apparatus and a receiving circuit according to the present invention comprise a front-end for converting a digital broadcast signal into an intermediate frequency signal, the digital broadcast signal having been modulated in orthogonal frequency multiplexing method of which data of each frame is composed of a synchronous reference signal and transmission data and of which data is assigned to a plurality of carriers that are orthogonal to each other, demodulating means for demodulating the intermediate frequency signal that is output from the front-end into a base band signal corresponding to the synchronous reference signal that is output from detecting means, detecting means, disposed in the demodulating means, for detecting the synchronous reference signal of each frame from the intermediate frequency signal that is output from the front-end, equalizing means, disposed in the demodulating means, for removing a signal component received through multi-paths from the synchronous reference signal, and outputting means for outputting the signal demodulated by the demodulating means.
The equalizing means delays the synchronous reference signal that is output from the detecting means corresponding to the time difference between the receiving timing of the synchronous reference signal received through a path of a direct wave and the receiving timing of the synchronous reference signal received through a path of a reflected wave and subtracts the synchronous reference signal that is delayed corresponding to the ratio between the reception level of the synchronous reference signal received through the path of the direct reflected wave and the reception level of the synchronous signal received through the path of the reflected wave and that is output from the detecting means and the synchronous reference signal that is output from the detecting means.
In addition, the equalizing means has a delay circuit portion for delaying the reference signal that is output from the detecting means corresponding to a delay amount that is set corresponding to the time difference, a gain setting portion for setting a gain corresponding to the ratio of the reception levels, and a subtracting portion for subtracting the synchronous reference signal that is output from the delay circuit portion through the gain setting portion from the synchronous reference signal that is output from the detecting means.
The demodulating means has a detecting portion for orthogonally detecting the intermediate frequency signal that is output from the front-end, a signal processing portion for performing a fast Fourier transform process for an output signal of the orthogonal detecting portion through the equalizing means, and a Viterbi decoder to which an output signal of the signal processing portion is supplied.
The receiving apparatus and the receiving circuit further comprise a frequency controlling circuit, disposed between the detecting means and the equalizing means, for receiving the synchronous reference signal from the detecting means, obtaining the frequency error of at least the synchronous reference signal with an output signal that is fed back from the signal processing portion, and controlling the frequency of the synchronous reference signal that is output from the detecting means.